Magnetic tapes have found various applications in audio tapes, videotapes, computer tapes, etc. In particular, in the field of magnetic tapes for data-backup (or backup tapes), tapes having memory capacities of several tens GB or more per one reel are commercialized in association with increased capacities of hard discs for back-up. Therefore, it is inevitable to increase the capacity of this type of tape for data-backup. It is also necessary to increase the feeding speed of tape and the relative speed between the tape and heads in order to quicken the access speed and the transfer speed.
To increase the capacity of tape for data-backup per one reel, it is necessary to increase the length of tape per reel by decreasing the total thickness of the tape, to decrease the thickness demagnetization so as to shorten the recording wavelength by forming a magnetic layer with a very thin thickness of 0.3 μm or less, and to increase the recording density in the tape widthwise direction by narrowing the widths of the recording tracks to 21 μm or less, particularly 15 μm or less.
When the thickness of the magnetic layer is reduced to 0.3 μm or less, the durability of the tape tends to be reduced. Therefore, at least one primer layer is provided between a non-magnetic support and the magnetic layer. When the recording wavelength is shortened, the influence of spacing between the magnetic layer and the magnetic heads becomes serious. Thus, if the magnetic layer has large projections or dents, which leads to a decrease in output due to spacing loss, the error rate increases.
When the recording density in the tape-widthwise direction is increased by narrowing the width of the tracks to 21 μm or less, particularly 15 μm or less, magnetic flux leaking from the magnetic tape is decreased. Therefore, it is necessary to use MR heads which utilize magnetoresistance elements capable of achieving high output from very small magnetic fluxes, for reproducing heads.
Examples of the magnetic recording media used in combination with MR heads are disclosed in JP-A-11-238225, JP-A-2000-40217 and JP-A-2000-40218. In these magnetic recording media, skewness of output from the MR heads is prevented by controlling the magnetic flux from the magnetic recording medium (a product of a residual magnetic flux density and the thickness of the medium) to a specific value, or the thermal asperity of the MR heads is suppressed by reducing the dents or projections on the surface of the magnetic layer to a specific value or less.
When the width of the tracks is decreased, the reproduction output lowers due to off-track. To avoid such a problem, track servo is needed. Types of such track servos include an optical servo type and a magnetic servo type. In either of these types, the track servo is performed on a magnetic tape drawn out from a magnetic tape cartridge (which may be also called a cassette tape) of a single reel type which houses only one reel having the magnetic tape wound thereon in a box-like casing body. The reason for using a single reel type cartridge is that the tape can not be stably run in a two-reel type cartridge which has two reels for drawing out the tape and for winding the same, when the tape-running speed is increased (for example, 2.5 m/second or higher) so as to quicken the data transfer speed. The two-reel type cartridge has another problem in that the dimensions of the cartridge become larger and that the memory capacity per volume becomes smaller.
As mentioned above, there are two types of track servo systems, i.e., the magnetic servo type and the optical servo type. In the track servo type, servo bands (200) as shown in FIG. 9 are formed on a magnetic layer by magnetic recording, and servo tracking is performed by magnetically reading such servo bands. In the optical servo type, servo bands each consisting of an array of dents is formed on a backcoat layer by laser irradiation or the like, and servo tracking is performed by optically reading such servo bands. Other than these types, there is such a track servo in which magnetic servo signals are also recorded on a magnetized backcoat layer in the magnetic serve type. Further, as other optical servo types, there is one which can record optical servo signals on a backcoat layer which is formed of a material capable of absorbing light or the like.
Then, the principle of the track servo system is simply described by way of the former magnetic servo type.
As shown in FIG. 9, in the magnetic tape (3) for the magnetic servo type, servo bands (200) for track serve which extend along the lengthwise direction of the tape, and data tracks (300) for recording data thereon are formed on the magnetic layer. Each servo band (200) consists of a plurality of servo signal-recording sections (201) on which the respective servo track numbers are magnetically recorded. A magnetic head array (not shown) which records and reproduces data on and from a magnetic tape consists of a pair of MR heads for servo track (forward running and backward running), and for example, 8×2 pairs of recording-reproducing heads (in which the recording heads are magnetic induction type heads and the reproducing heads are MR heads). In response to a signal from a MR head for servo track which has a read servo signal, the entire magnetic head array moves interlocking therewith, so that the recording-reproducing head moves in the widthwise direction of the tape to reach the data track (for example, eight data tracks are provided corresponding one serve track in a magnetic head array on which 8×2 pairs of recording-reproducing heads are mounted).
In this stage, the magnetic tape runs in such a state that one of the tape edges extending along the lengthwise direction is regulated in its tape widthwise position by the inner surface of a flange of a guide roller provided on a magnetic recording-reproducing unit (a tape-driving unit) (see FIG. 7). As seen in FIG. 3, the edge (3a) of the magnetic tape (3) generally has a corrugated unevenness called edge weave or edge wave. Therefore, the magnetic tape (3), even though running alongside the inner surface of the flange as the reference for the tape running, very slightly fluctuates in the position in the widthwise direction. However, this problem is solved by employing the above servo system: that is, even if the position of the magnetic tape very slightly fluctuates in the widthwise direction, the entire magnetic head array moves in the tape widthwise direction in association with such a fluctuation, so that the recording-reproducing head can always reach a correct data track. In a system for recording tracks with widths of 24 μm or more, the off-track margins are increased by widening the width of the recording track in comparison with the width of the reproducing track [for example, (the width of the recording track: about 28 μm, and the width of the reproducing track: about 12 μm) or (the width of the recording track: about 24 μm, and the width of the reproducing track: about 12 μm)]. In such a case, there arises little decrease in the reproducing output due to off-track, even when about 3 μm of fluctuation (edge weave) in the position of the magnetic tape occurs.
However, when the width W of the record track is reduced to 21 μm or less, a decrease in output of reproduction due to off-track appears in spite of about 3 μm of edge weave which raised no problem in the conventional record tracks. This is because, when the reproduction track width is equal to the conventional one in order to ensure a reproduction output, the off-track margin becomes narrower. Further, when the recording track width is as narrow as 21 μm or less, it is confirmed that not only the absolute value of edge weave but also the cycle of the edge weave and the tape running speed have a complicated relationship with respect to the off-track. To apply the servo system to a magnetic tape having record tracks with a width as narrow as 21 μm or less, a relationship among the cycle f and the amount a of edge weave, the record track width W, the tape running speed V, and the head followability is carefully examined. As a result, the following are revealed: a positioning error signal (or PES, i.e. a value indicating non-uniformity in positional dislocation, or the value of a standard deviation σ) becomes larger, if the values of α/W and (α/W)×(V/f) exceed specific values, wherein α is an amount of the edge weave (in the tape widthwise direction of the tape edge (the direction Y-Y′ on FIG. 3)) with a cycle of f; V [mm/second] is a tape running speed; and W [μm] is a width of the record track. As a result, a tracking error is induced. This phenomenon raises a new problem when the width of the record track is set at 21 μm or less.
This is described below. Since the magnetic head array as a whole has large mass, the magnetic head array can not move following the motion of the magnetic tape in the widthwise direction, when the values of (α/W) and/or (α/W)×(V/f) exceed specific values, wherein α is an amount of the edge weave with a cycle of f on an edge of the tape (not only one edge (3a) of the tape as shown in FIG. 3, but also both edges (3a, 3a′) of the tape as described below) regulated in its position while the tape is running; W is a width of the record track; and V is a tape-running speed. As a result, a positioning error signal or PES becomes larger. When the off-track margin is small, it is presumed that the off-track becomes larger. This phenomenon is not so serious when the width of the record track is 24 μm or more. Why this is not so serious is that, if the motion of the magnetic head array is slow and the PES is large, the width of the record track is sufficiently larger than the width of the reproducing track, so that the off-track margin is large: for example, about 6 μm or more of off-track margin is formed on each side, when the width of the record track is about 28 μm, and the width of the reproducing track is about 12 μm, or when the width of the record track is about 24 μm, and the width of the reproducing track is about 12 μm. Therefore, a decrease in the output of reproduction due to off-track seldom occurs.
As mentioned above, in the magnetic recording-reproducing unit (i.e., the tape-driving unit), the width of the groove of the guide roller (the distance between the inner surfaces of a pair of flanges provided on the both edges of a guide roller, see FIG. 7) is set at a dimension several ten micrometers larger than the width of the magnetic tape. Therefore, the cycle f and the amount α of the edge weave as the reference side for running are dominant over the linearity of a serve signal. On the other hand, in a unit for recording serve signals (a serve writer), the width of the groove of the guide roller is set at a dimension substantially equal to the width of the magnetic tape so that there is little clearance. Therefore, both tape edges (3a, 3a) of the tape serve as the reference sides for tape running, and thus, the cycle f and the amount α of the edge weaves of both tape edges (3a, 3a) are dominant over the linearity of the servo signal. Therefore, to decrease the off-track by decreasing PES, the relationship among the cycle f and the amount α of the edge weaves of both tape edges (3a, 3a), the width W of the record track, and the tape-running speed V should satisfy the above equation.
When the width of the record track is as narrow as 21 μm and the level of PES becomes larger, an off-track error occurs, so that a normal servo control can not be preformed. Such a problem commonly arises in both of the magnetic servo type and the optical servo type, and it is more remarkable in the optical servo type, because the mass of the entire magnetic head array used in the optical servo type is larger than that used in the magnetic servo type.